Polylactic acid is a well known biodegradable material. Since lactic acid can be derived from renewable resources, the development of lactic acid industry can reduce the demand for petroleum resources and is also beneficial to mitigate greenhouse effect. However, polylactic acid is brittle and has an insufficient toughness. Thus an important aspect for improving the mechanical properties of polylactic acid is to toughen the polylactic acid.
Copolymerization is an effective method to toughen polylactic acid. CN1911983A, for example, disclosed a preparing method of a degradable polymer material for shape memory endoluminal stents, wherein two of lactide, glycolide and caprolactone are ring-opening polymerized to provide a hydroxyl-terminated oligomer; and the resulting hydroxyl-terminated oligomer and a linear polyester are subjected to bulk polymerization so as to produce a degradable polymer material; wherein the linear polyester is a polycondensation product of an aliphatic diol and an aliphatic dicarboxylic acid or an aliphatic dicarboxylic anhydride.
Although methods known in prior art, which subject polylactic acid to copolymerization, can improve the toughness of polylactic acid, these methods can only produce opaque polylactic acid resin materials. The loss of the excellent inherent transparency of polylactic acid limits the applications of these polylactic acid materials greatly.
In order to produce a toughened polylactic acid material with transparency, a common way is to blend polylactic acid, a plasticizer, and a toughening agent, and so on. For example, it is disclosed in CN1673276A that a transparent polylactic acid can be prepared by blending polylactic acid, glycerol triacetate, a lubricant and so on; it is disclosed in CN101314666A that a transparent polylactic acid can be obtained by blending polylactic acid, a nanosilica, an amide compound and so on; it is disclosed in CN1687205A that a completely degradable, transparent and toughened polylactic acid material can be obtained by blending polylactic acid, polyvinyl alcohol, epoxidized soybean oil and so on; and it is disclosed in CN101983986A that a transparent and toughened polylactic acid material can be obtained by blending polylactic acid, a methyl methacrylate-butyl acrylate copolyester as toughening agent, a plasticizer and an antioxidant and so on.
However, polylactic acid resin materials prepared by blending suffer problems such as phase separation, bleeding of plasticizers, and so on. Therefore it is desirable to provide a transparent and toughened polylactic acid material without defect(s) of prior art.